Methods and apparatus for coating articles by static electricity



Dec. 3, 1963 TAMOTSU WATANABE 3, 37

METHODS AND APPARATUS FOR comm; ARTICLES BY STATIC ELECTRICITY Filed Sept. 19, l960 2 Sheets-Sheet 1 @IEIEZI 1:1 6

0 O O O [5] [3'0 o o o INVENTOR Tamofsu Waranabe ATTORNEYS Dec. 3, 1963 TAMOTSU WATANABE 7 METHODS AND APPARATUS FOR COATING ARTICLES BY STATIC ELEQTRICITY Filed Sept. 19, 1960 2 Sheets-Sheet 2 INVENTOR Tamoisu Waranabe ATTORNEYS United States Patent 3,113,037 METHODS AND APPARATUS FUR CUATING ARTICLES BY STATIC ELECTRICITY Tamotsu Watanabe, 28 Yasukata-cho, Ohta-Ward, Tokyo, Japan Filed ept. 19, 196i), Ser. No. 56,976 7 Claims. (Cl. 117-93.4}

The present invention relates to improvements in electrostatic coating methods and apparatus, and which causes all of the atomized and discharged coating material to be deposited on the surface of objects to be coated.

In known electrostatic coating methods, using either liquid or powder coating material, and wherein atomization of the coating material is effected either by means of a cup-shaped electrode head, by air pressure, or by pressure applied directly to the coating material, when the atomized coating material is charged with a high potential and, due to electrostatic field action, is moved toward the work pieces or objects to be coated, it could not be expected that all of the atomized coating material particles would be deposited effectively on the work piece surfaces. it is unavoidable that considerable quantities of the coating material particles escape from the coating zone through gaps between the work pieces to be coated, and this escaped material has been accepted as an inevitable coating material loss. However, the escaped material is not only a cause of loss of efficiency of the coating operation, but also a cause of interference due to the escaped material generally depositing on walls, floors, pillars and the like surrounding the work pieces because the latter are usually at the same potential as the work pieces.

It should be explained that, in electrostatic coating methods iii-thereto known, the work pieces are generally at ground potential so that, during the coating operation the atomized particles charged to a potential quite considerably above ground potential, as by virtue of a high potential applied to the atomizer, move toward the objects to be coated which are at ground potential and deposit thereon. However, inasmuch as other parts around the work pieces, such as the walls of spraybooths and the like, are also at ground potential, those coating material particles bypassing the work deposit upon other grounded objects. This not only results in poor economy of the coating operation, but is disadvantageous with respect to coating operations due to accumulation of coating material in the work area.

The principal object of this invention is to cause all of the coating particles, even those escaping through gaps, between the work pieces that are discharged to impinge objects to be coated while the latter are moving through an effective coating zone, and to absolutely prevent coating material loss and deposition on other objects.

Another object of this invention is to keep the workshop clean by avoiding deposition of coating particles on areas other than objects to be coated, making agreeable operations possible even in a relatively narrow room.

Another object of the invention is to provide a method for efiicient and economic performance of operations by charging discharged coating particles with a negative potential, by virtue of a negative voltage applied to the supply means, while objects to be coated, which constiice tute a counter pole, are charged with a positive voltage during a period when they are positioned in etfective coating zone, being carried by one and same conveyor means which automatically continues the abovementioned operation.

Still another object of the invention is to further improve the aforementioned coating operation by eliminating any operational hazard by discharging any electrostatic charge from objects to be coated entering a coating zone, called the effective coating zone, applying a high voltage charge to the objects to be coated while they are passing in front of the coating material spraying or discharging means, and grounding the coated objects as they leave the effective coating zone to discharge any remaining electrostatic charge thereon.

Additional object of this invention is to ensure an operation absolutely safe from any electrical hazard as far as objects to be coated are concerned when they are out of said effective coating zone, by preventing discharge phenomenon liable to occur at a moment when work pieces charged to a high voltage enter and pass out of the specific effective coating zone, the application of high voltage being restricted to the efiective coating zone only.

Another additional object of this invention is to realize satisfactory coating over the whole surface of objective articles, and the back surface also, even in a single-faced coating operation, by assuring those coating particles that fly off around objects to be coated shall certainly be drawn back into the effective coating zone and toward objects to be coated, and to deposit thereon.

A further important object of the invention is to effect electrostatic coating operations more safely and easily than is possible with methods used heretofore, by' maintaining a low potential difference between a voltage of one polarity applied to the atomized particle supply means and a voltage of another polarity applied to the work pieces but only within the effective coating zone.

Another object of the present invention is to avoid unsatisfactorily coated surfaces due to deposition of minute foreign material present in the working area 'by virtue of applying potential to the work pieces only within the effective coating zone and by removing any charge from the work pieces before the latter enter the effective coating zone and immediately as they pass out of the effective coating zone.

For an understanding of the principles of the invention, reference is made to the following description of a ,typical embodiment thereof as illustrated in the accompanying drawings. In the drawings:

FIG. 1 is a perspective view of one form of apparatus embodying the invention;

FIG. 2 is a schematic block and electric diagram illustrating the principles of the invention;

FIG. 3 is a schematic elevational view and electric diagram illustrating the electrical circuit operation as the work pieces enter and leave the effective coating zone;

FIG. 4 is a plan view, partly in section, illustrating the operation of auxiliary rails;

FIG. 5 is an end elevational view, partly in section, illustrating the action of the auxiliary rails; and

FIG. 6 is a schematic wiring diagram illustrating the switching arrangement.

The coating apparatus 16 according to the present invention comprises, as shown in FIG. 1, for instance, a suitable number of nozzles 11, which discharge a fog of liquid coating material, these nozzles being arranged on the peripheral face of rotatable atomizing head 12, the nozzles being so directed as to discharge the spray therefrom in a direction opposite to the direction of rotation of the atomizing head 12. The head 12 is electrically isolated from the grounded coating apparatus 19 and is connected to the negative output terminal of a high voltage source 14 whose casing or chassis is grounded. The support for the rotating head 12 is also grounded. The work pieces 15 are moved on the conveyor 16 having hooks 17 from which depend suspension rods 20. Each suspension rod includes an insulator 18 and an electrically conductive portion 19, and the work pieces or articles to be coated are suspended from hooks at the lower ends of the rods 20.

At the lower part of conveyor 16 that moves suspended objects to be coated 15 is securely arranged an electrically conductive rail 21 which is maintained in contact with conductive portions 19 of suspending rods 20 by means of conventional insulated supports 22, 23, 24. At the same time, on both ends of rail 21 there are provided insulating portions 25 and 26 of some length, with grounding rails 27 and 28 extending therefrom toward tips. The whole of the rail is supported securely, as mentioned above at suitable portions by the insulated supports, and the rail 21 is connected with positive voltage output terminal 29 of high-voltage producer 14-. Grounding rails 27 and 28 are suitably grounded. Auxiliary rails 30 and 31 are mounted so as to project slightly from a vertical line, by means of insulated supports 36 and 37, directly below insulating portions 25 and 26 on both ends of rail 21 and parallel thereto. Auxiliary rails 31) and 31 are insulating or dielectric sections 34 and 35 extending outwardly from their midpoint, and electrically conductive sections 32 and 33 extending inwardly from their midpoint. The midpoints between the insulating sections and the conductive sections of the two rails are substantially vertically aligned with the midpoints of the respective insulating sections 25 and 26. The center sections of auxiliary rails 30 and 31 are substantially flat, in plan, as best seen in FIG. 4. The rails are slidably supported in insulating supports 36 in which are positioned suitable microswitches 38 and 39.

As each auxiliary rail 30 or 31 is engaged and pressed inwardly by a work piece or its support, the associated switch 38 or 39 is operated in one direction, as each rail 30 begins to move outwardly again under a spring bias, the associated switch 38 or 39 is operated in the reverse direction. These switch operations, as will be described, control connection of conductive section 32 or 33 to the same source of potential to which the rail 21 is connected. Each switch 38 or 39 controls energization of an operating coil in an oil immersed switch 40. When the associated switch 33 or 39 is in its normal closed position, the associated operating coil is energized to switch contacts and, when a switch 38 or 39 is operated by movement of rail 30 or 31 due to passage of a work article thereby, the associated switch will de-energize its connected relay coil.

It will be noted, from FIG. 4, that each rail section 32 or 34, oi rail 30, and 33 or 35, of rail 31, includes portions inclined, in plan, to the direction of travel of the work articles and to the rail 21. Thus, as the suspending rod 20 for a work article engages, for example, the inclined portion of section 34, the associated rail 30 will be moved inwardly and will be maintained in the inward position until the rod 20 has passed the point Y. As the rod 20 then moves along the inclined section 32, the rail .30 again moves outwardly. The same occurs with respect to the point Y of the rail 31.

Further, in FIG. 2, P represents coating material tank, Q compressed'air supply means, and M indicates a motor.

The apparatus thus far described operates in the following manner:

Conveyer means 16 continually moves objects to be coated 15 in a suspended manner the same as in known electrostatic coating methods. However, as the suspended objects 15 are supported in insulated relation to ground, and as the suspending elements are insulators, the method of this invention is entirely different from known methods. When rods 20 reach conduction rail 21, each suspending rod 20 slides first on grounding rail 27, and then on insulating portion 25 and conduction rail 21 successively, and at the rear end of conduction rail 21, each rod 2%) slides on insulating portion 26 and grounding rail 23, after which objects 15 leave the rail portions. When conduction portion 19 of suspending rod 26 contacts with conduction rail 21 in the order mentioned above, positive high voltage is applied instantly to objects to be coated, simultaneously forming an electric field between atomizing head 12, arranged in front of rail 21s position, and the articles to be coated. All of the coating particles discharged from atomizing head 12, being charged negatively, fly toward and deposit on objects to be coated 15, which are positively charged. When rod 20 reaches the insulator 26 after'passing conduction rail 21 and comes in contact with grounding rail 28, any charge remaining in objects 15 is drawn 011 and they return to the electrically isolated suspended state.

In the apparatus according to this invention, the auxiliary rail means 30 and 3-1 are provided at both ends of conduction rail 21 in order to prevent sparks which 0C cur at an instant when suspending rods 20 move from insulating portion 25 to rail 29', or move from rail 20 to insulating portion 26. With reference to auxiliary rail 31 on the side where suspending rod 2% enters, when conduction portion 19 of suspending rod 20 initially contacts grounding rail 27, objects are caused to become the same potential as the ground, and then portion 19 contacts insulating portion 34 of auxiliary rail 30 and insulating portion 25' of conduction rail '21. The moment it reaches the first activating point X, suspending rod 20 causes auxiliary rail 3% to be moved inwardly due to the gravity of the suspended objects 15, portion -19 being also in contact with insulating rail section 25. Switch 38 will thereby be operated to effect disconnection of conductive section 32 from the potential of rail 21. There will be no spark as the rod 26* passes from section 34 to section 32. When rod 20 reaches the activating point Y, and before portion 1 9 has disengaged insulated rail insert 25, it will proceed along the sloping portion of rail 32 and rail 30 will thus move outwardly and, after a mornent, switch 38 will be operated to re-connect conductive section 32 to the potential of rail 21, again before portion 13 has left rail insert 25. Hence, as the rod 29 disengages the insulated rail insert 25 and engages conductive rail 21, there will be no spark because rod 26, due to engagement with rail section 32, will be already at the potential of rail 21.

Now, referring to auxiliary rail 31 on the side where suspending rod 20 passes out of the effective coating zone, suspending rod 20, before reaching the rear end of conduction rail 21, comes in contact with conductive section 33 of auxiliary rail 31, and then reaches actuating point X. Due to the inward movement of rail section 31, switch 39 is operated to disconnect conductive section 33 from the potential of rail 21 after portion 1-9 of rod 20 engages insulating insert 26. Previously, there will have been no spark as the rod 29 came into engagement with conductive section 33 because this is at the same potential as rail 21. Conductive section 33 is now disconnected from the potential of rail 21 so that there is no spark as rod 20 passes from: section 33 to insulated section 35. As rod 24? reaches the second point Y, rail 31 begins to move outwardly and, as the rod 28 contacts grounding section 28, or immediately therebefore, switch 39 is operated by such outward movement of rail 31 to re-connect conductive portion 33 to the potential of rail 21. As rod 20 will have already disengaged section 3-3 at this time, and is in engagement with insulating rail portion 35 and insulating rail insert 26, there is no spark. The rod 20 then engages the grounding section 28 so that any charge remaining on the article is drawn ed to ground as the latter leave the effective coating zone.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

What I claim is:

1. An electrostatic coating method comprising the steps of moving articles to be coated along a path including an effective coating zone, while maintaining the articles insulated from ground; atomizing coating material in the effective coating zone to form atomized particles of coating material; applying to the atomized particles the polarity and potential of one polarized output terminal, having a potential difference with respect to ground, of a source of relatively high DC. potential; just in advance of the entry of the articles into such effective coating zone, momentarily grounding each article; while the articles are in the elfective coating zone, connecting the articles to the opposite polarity polarized output terminal, having a potential difference with respect to ground, of such source, to move substantially all the atomized particles only onto the articles to be coated; and momentarily grounding each coated article as it leaves such eiiective coating zone to remove any charge therefrom.

2. An electrostatic coating method, as claimed in claim 1, including the step of, following such grounding of the articles immediately in advance of their entry into the effective coating zone, again insulating each article from ground before connecting the same to such opposite polarity polarized output terminal.

3. An electrostatic coating method, as claimed in claim 1, including the step of, in advance of grounding the coated articles as they leave such cfiective coating zone, disconnecting each article from such opposite polarity polarized output terminal.

4. Electrostatic coating apparatus comprising, in combination, a source of relatively high DC. potential having a pair of opposite polarity polarized output terminals having oppositely directed potential difierences with respect to ground; conveyor means for moving articles to be coated along a path including an effective coating zone; means tor supporting the articles on said conveyor means in insulated relation to ground, said supporting means including an electrically conductive portion "connected to the respective supported articles; an electrically conductive rail extending longitudinally of said zone and arranged to be contacted by the conductive portion of each supporting means as the latter passes through said zone; a coating material atomizing head having an atomized coating material particle discharge portion electrically connected to one of said polarized output terminals; means supporting said rail in insulated relation to ground; means electrically connecting said rail to the other of said polarized output terminals; whereby, as each article enters such effective coating zone, it is charged to the polarity and potential of said rail for movement of substantially all of the atomized particles only onto the articles to be coated passing through said coating zone; a pair of insulated rail inserts each extending from a respective end of said electrically conductive rail; and a pair of grounded terminal rail sections each extending from a respective outer end of one of said insulating rail inserts; whereby, as each uncoated article approaches the effective coating zone, it is momentarily grounded while the electrically conductive portion of its supporting means is in contact with the associated grounded rail section and, after each coated article leaves the effective coating zone, it is grounded by the electrically conductive portion of its supporting means engaging the other grounded rail section, to remove any charge from the coated article.

5. Electrostatic coating apparatus, as claimed in claim 4, including a pair of auxiliary rails each adjacent an end of said electrically conductive rail and extending in spaced substantially parallel relation thereto for engagement by the electrically conductive portion of the supporting means for the articles; switch means normally operable to connect each auxiliary rail to said electrically conductive rail; means, including the leading auxiliary rail, adjacent the leading end of said electrically conductive rail, efiective, in advance of electrical connection of said leading auxiliary rail to the electrically conductive portion of each supporting means, to operate the associated switch means to disconnect said first-named auxiliary rail from said electrically conductive rail and, while such electrically conductive portion of each supporting means is electrically connected to said leading auxiliary rail and not yet engaged with said electrically conductive rail, to re-connect said auxiliary rail to said electrically conductiverail; and means, including the trailing auxiliary rail, adjacent the trailing end of said electrically conductive rail, effective, when the latter is electrically connected to the electrically conductive supporting means portion of each supporting means and while such portion is still engaged with said electrically conductive rail, to operate the associated switch means to disconnect said trailing auxiliary rail from said electrically conductive rail and, after the electrically conductive portion of each supporting means is electrically disconnected from both said trailing auxiliary rail and said electrically conductive rail, to operate said switch means to re-connect said trailing auxiliary rail to said electrically conductive rail.

6. Electrostatic coating apparatus as claimed in claim 5, each auxiliary rail extending through the length of said insulating rail insert and overlapping the associated end of said conductive rail and the inner end of the associated grounded rail section; each auxiliary rail being spring biased to project somewhat beyond the vertical plane of the electrically conductive rail; said switch means including normally closed microswitch means opened upon inward movement of the associated auxiliary rail; the leading auxiliary rail being divided intermediate its ends into a leading section of dielectric material and a trailing section of electrically conductive material; the trailing auxiliary rail being divided, intermediate its ends, into a leading section of electrically conductive material and a trailing section of dielectric material; whereby, as the electrically conductive portion of each article supporting means is in engagement with the leading grounded rail section, it will engage the dielectric section of the leading auxiliary rail and, while in engagement therewith, will engage said insulating rail insert; the electrically conductive portion of each supporting means, while still in engagement with said insulating rail insert, then engaging said electrically conductive trailing section of the leading auxiliary rail and remaining in contact therewith until after it has come into engagement with said electrically conductive rail; the electrically conductive portion of the supporting means for each article, as it approaches the trailing end of said electrically conductive rail, engaging the leading conductive section of said trailing auxiliary rail and remaining in contact therewith while passing from said electrically conductive rail to said insulating rail insert and, while still in engagement with said insulating rail insert, engaging the trailing dielectric section of said trailing auxiliary rail and remaining in contact therewith until after it has engaged the trailing grounded rail section; each auxiliary rail being moved inwardly by an article supporting means, to open the associated switch means, only while the elec trica-lly conductive portion of the supporting means is in engagement with the associated insulating rail insert.

7. Electrostatic coating apparatus, as claimed in claim 6, in which each auxiliary rail, as viewed in plan, has sloping entry and exit sections whereby, as a supporting means therealong, the associated auxiliary rail is moved inwardly and then permitted to move outwardly under its spring bias; the switch means associated with each auxiliary rail being operated to efiect disconnection thereof from said electrically conductive rail before an article supporting means has engaged the trailing section of the associated auxiliary rail and being operated to re-connect the associated auxiliary rail to said electrically conductive rail after a supporting means is in engagement with the trailing section of its associated auxiliary rail.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTROSTATIC COATING METHOD COMPRISING THE STEPS OF MOVING ARTICLES TO BE COATED ALONG A PATH INCLUDING AN EFFECTIVE COATING ZONE, WHILE MAINTAINING THE ARTICLES INSULATED FROM GROUND; ATOMIZING COATING MATERIAL IN THE EFFECTIVE COATING ZONE TO FORM ATOMIZED PARTICLES OF COATING MATERIAL; APPLYING TO THE ATOMIZED PARTICLES THE POLARITY AND POTENTIAL OF ONE POLARIZED OUTPUT TERMINAL, HAVING A POTENTIAL DIFFERENCE WITH RESPECT TO GROUND, OF A SOURCE OF RELATIVELY HIGH D.C. POTENTIAL; JUST IN ADVANCE OF THE ENTRY OF THE ARTICLES INTO SUCH EFFECTIVE COATING ZONE, MOMENTARILY GROUNDING EACH ARTICLE; WHILE THE ARTICLES ARE IN THE EFFECTIVE COATING ZONE, CONNECTING THE ARTICLES TO TO OPPOSITE POLARITY POLARIZED OUTPUT TERMINAL, HAVING A POTENTIAL DIFFERENCE WITH RESPECT TO GROUND, OF SUCH SOURCE, TO MOVE SUBSTANTIALLY ALL THE ATOMIZED PARTICLES ONLY ONTO THE ARTICLES TO BE COATED; AND MOMENTARILY GROUNDING EACH COATED ARTICLE AS IT LEAVES SUCH EFFECTIVE COATING ZONE TO REMOVE ANY CHARGE THEREFROM. 